Metal matrix composites (MMC) have been fabricated since the early 1960's primarily by powder metallurgy techniques. Recently, however, liquid methods have been more frequently utilized due to a great saving in manufacturing costs. The most common approach of producing shaped MMC products via the liquid method is to infiltrate open spaces of packed aggregate beds or preforms with molten matrix.
At the present time, the processes employed for fabrication of Al base MMC include a vacuum/low pressure infiltration, a high pressure squeeze casting process, and a pressureless casting process. These processes have the following characteristics:
Low pressure/vacuum infiltration process--The process utilizes a combination of vacuum and a low pressure (less than 1500 psi) gas for metal infiltration. The process has been employed mainly for fabrication of small numbers of R&D samples. It is designed to be a laboratory scale process and is not suitable for low cost, high throughput production in commercial scale.
Pressureless infiltration process--The process relies on capillary force for the infiltration. To develop the capillary force, a wetting between aggregate and matrix materials is obtained through a chemical reaction. The process requires the use of special combinations of matrix alloying element (aluminum with 1 w % (The terms "v %" and "w %" as used throughout this specification mean volume-% and weight-%, respectively.) Mg or higher), heated ceramic aggregate (700 C. or higher), and environment (10 to 100 v % nitrogen). This process thus limits selection of matrix material, for instance to Al-Mg alloys, and is burdened by high manufacturing costs due to extended reaction periods. This process is additionally difficult from a process control standpoint due to the sensitive dependence of the process on surface condition and spontaneous wetting.
High Pressure Squeeze Casting Process--For the infiltration, this process utilizes only high pressure (10,000 to 30,000 psi) using a large hydraulic or mechanical press. The process has the ability to infiltrate a wide range of aggregate and matrix combinations but is quite limited as to complexity and dimensional tolerance of the products produced because of limitations in die design imposed by the needs for high pressure. The product manufacturing costs are also high because of high capital costs for the needed large press.
U.S. Pat. No. 4,920,864 mentions a closed-die pressure casting process to achieve infiltration of a matrix alloy into the interstices of a fiber preform, but does not provide any further information. U.S. Pat. No. 4,777,998 shows die casting of metal parts possibly containing fibers of ceramic materials, but no attempt is made to utilize vacuum die casting principles.